The present disclosure relates to a signal transmission apparatus, an electronic device, and a signal transmission method, and particularly to modulation distortion (“intermodulation distortion” in particular) occurring due to the nonlinearity of a circuit member.
Not only transmission between one set of a transmitter and a receiver using one carrier frequency but also signal transmission with a combination of a plurality of frequencies may be made within a communication area. For example, there are a case in which a frequency division multiplexing system is applied (two carrier frequencies are used) and bidirectional simultaneous communication is performed between one set of communicating devices, and there is a case in which communication (unidirectional communication or bidirectional communication) is performed with respective different carrier frequencies used between a plurality of communicating devices. There is also a case in which one set of communicating devices has a plurality of sets of modulating circuits and demodulating circuits and makes multicarrier (MC) transmission as one of methods for lowering a symbol rate, as typified by OFDM (Orthogonal Frequency Division Multiplexing) transmission.
In any case, regardless of the number of sets of communicating devices, a plurality of sets of modulating circuits and demodulating circuits are provided, and simultaneous communication is performed with the sets of the modulating circuits and the demodulating circuits using respective different carrier frequencies. When transmission is made with a combination of the plurality of frequencies, modulation distortion occurring due to the nonlinearity (nonlinear operation) of a circuit member such as an amplifier, a frequency mixing section (mixer) or the like degrades reception quality. For example, when signals of two carrier frequencies totally unrelated to a desired wave (own station) are received and input to an amplifying circuit or a frequency mixing circuit having nonlinearity, a signal (disturbing wave component) of a difference between the two carrier frequencies is also output. At this time, when the difference between the two carrier frequencies is present in the vicinity of the frequency of the desired wave, there occurs a problem of “intermodulation distortion” in that the disturbing wave component is also demodulated. Typically, when signals of a plurality of frequencies adjacent to the reception band of the own station are received, and the amplifier or the frequency mixing section has poor linear performance, third-order distortion occurs within the reception band (it usually suffices to consider only the first-order component of a modulated signal), and greatly degrades reception quality.
For example, Japanese Patent Laid-Open No. Sho 55-38777 proposes a method of disposing the frequency of narrow-band modulation at a position of an integral multiple of the reception bandwidth of spectrum spreading modulation as a method for relaxing the performance of a receiver in a case of a combination of spectrum spread and narrow-band modulation. However, when a plurality of narrow-band modulations are used, the third-order distortion of the plurality of narrow-band modulations occurs within the band of the spectrum spreading modulation, and it is not possible to adapt to a plurality of spectrum spreading modulations.